1. Technical Field
The present invention relates to a method of depositing a noble metal on a structure member of a nuclear plant and more particularly to a method of depositing a noble metal on a structure member of a nuclear plant preferably applied to a boiling water nuclear plant.
2. Background Art
As a nuclear plant, for example, a boiling water nuclear power generation plant (hereinafter referred to as BWR plant) and a pressurized-water nuclear power generation plant (hereinafter referred to as PWR plant) are known. The BWR plant has a nuclear reactor having a core disclosed in a reactor pressure vessel (referred to as RPV). Cooling water supplied to the core by a recirculation pump (or an internal pump) is heated by the heat generated by nuclear fission of a nuclear fuel material in a fuel assembly loaded in the core and a part of the heated cooling water becomes steam. This steam is introduced to a turbine from the nuclear reactor and rotates the turbine. The steam discharged from the turbine is condensed by a condenser into water. This water is supplied to the RPV as feed water. The feed water suppresses generation of a radioactive corrosion product in the RPV, so that mainly metallic impurities are removed by a demineralizer installed in a feed water pipe.
In a nuclear plant such as a BWR plant and a PWR plant, the main structure members such as an RPV use stainless steel and nickel group alloy for a water contact portion in contact with water to suppress corrosion. A part of reactor water which is cooling water existing in the RPV is purified by the demineralizer of a reactor purification system and few metallic impurities existing in the reactor water are removed positively.
Even if the aforementioned corrosion measure is taken, it cannot be avoided that very few metallic impurities remain in the reactor water, so that a part of the metallic impurities, as a metallic oxide, is deposited on surface of fuel rods included in a fuel assembly. The deposited metallic impurities (for example, a metallic element) cause a nuclear reaction by irradiation of neutrons generated due to nuclear fission of a nuclear fuel material in the fuel rods and become a radioactive nuclide such as cobalt 60, cobalt 58, chromium 51, or manganese 54. These radioactive nuclides are mostly kept deposited on the fuel rod surface in an oxide shape, though a part of the radioactive nuclides elutes in the reactor water as ions according to the solubility of the taken-in oxide or are redischarged into the reactor water as an insoluble solid called clad. The radioactive material included in the reactor water is removed by the reactor water clean-up system.
However, the radioactive material that was not removed is stored on the surface of the structure member constituting the BWR plant in contact with the reactor water while it circulates in a recirculation system or the like along with the reactor water. As a result, the radiation radiated from the surface of the structure member causes a radiation exposure of an operator performing the periodic inspection of the BWR plant. Radiation exposure dose of the operator is controlled so as not to exceed the specified value for each person. Recently, the specified value has been decreased and the need is arising to decrease the radiation exposure dose of each person as much as economically possible.
So, a method of decreasing deposition of a radioactive nuclide on an inner surface of a pipe and a method of decreasing concentration of a radioactive nuclide in the reactor water are being discussed variously. For example, the following method has been proposed: Inject metallic ions such as zinc into the reactor water and on the inner surface of the recirculation pipe in contact with the reactor water to form a minute oxide film including zinc, thereby suppressing the take-in of a radioactive nuclide such as cobalt 60 or cobalt 58 into this oxide film (see Japanese Patent Laid-Open No. 58 (1983)-79196).
Japanese Patent Laid-Open No. 2006-38483 and Japanese Patent Laid-Open No. 2007-192745 describe a method comprising steps of bringing a processing solution including a formic acid aqueous solution including iron (II) ions, hydrogen peroxide, and hydrazine and the processing solution heated within the range from the normal temperature to 100° C. into contact with a surface of a structure member of a nuclear plant, forming a ferrite film on the surface, and suppressing deposition of a radioactive nuclide on the surface of the structure member after operation of the nuclear plant. Furthermore, a method has been proposed of forming a nickel ferrite film or a zinc ferrite film, which are more stable than a magnetite film, on a surface of a structure member of a nuclear plant, and furthermore suppressing deposition of a radioactive nuclide on the surface of the structure member after operation of the nuclear plant. It is described in Japanese Patent Laid-Open No. 2010-127788 that the forming quantity of the ferrite film formed on a surface of a structure member of a nuclear plant is measured by a quartz crystal microbalance apparatus.
Japanese Patent Laid-Open No. 2006-38483 describes that during operation stop of a nuclear plant, chemical decontamination of a surface of a structure member of a nuclear plant is executed, and a ferrite film is made to form on the surface, and platinum is made to deposit on the surface of the ferrite film.
To suppress stress corrosion cracking of a structure member of a nuclear plant, it is described in Japanese Patent Laid-Open No. 2001-91688 that a composite oxide layer of zinc and chromium in which zinc chromite (ZnCr2O4) and chromium oxide (Cr2O3) coexist is formed on a surface of a structure member in contact with the reactor water.
Chemical decontamination for removing an oxide film including a radioactive nuclide formed on a surface of a structure member of a nuclear plant that experienced an operation in contact with the reactor water, for example, on an inner surface of a pipe connected to a reactor pressure vessel is described in Japanese Patent Laid-Open No. 2000-105295. This chemical decontamination is executed to decrease the radiological dosage. Chemical decontamination mainly includes an oxidation decontamination process in which an oxidation decontamination solution is used, an oxidation decontamination agent decomposition process, a reduction decontamination process in which a reduction decontamination solution is used, a reduction decontamination agent decomposition process, and a purification process.
Nuclear plants apply technique of injecting hydrogen and platinum into the reactor water in the RPV to suppress the progress of stress corrosion cracking (see proceedings of water chemistry 2004, pages 1054-1059). In the platinum injection technique, concretely, a platinum complex is injected from the feed water pipe into the RPV after start of the nuclear plant and the platinum is deposited on the surface of the structure member (for example, the pipe of the nuclear plant) of the nuclear plant in contact with the reactor water. As a result, the surface is kept in a strong reduction environment and the progress of the stress corrosion cracking of the structure member is suppressed.